An ice cream shop has the following flavor and topping options: For flavors, they have Vanilla, Chocolate, and Cookie Dough. For toppings, they have Sprinkles, Cherries, and Caramel. If a group of students order ice cream (each with one flavor and one topping), what's the smallest number of people that must be in the group to guarantee that there are at two people in the group that ordered the same flavor-topping combination? Explain.